Myosin II assembly and localization into the cytoskeleton is regulated by heavy chain phosphorylation in Dictyostelium. The enzyme myosin heavy chain kinase A (MHCK A) has been shown previously to drive myosin filament disassembly in vitro and in vivo. MHCK A is noteworthy in that its catalytic domain is unrelated to the conventional families of eukaryotic protein kinases. We report here the cloning and initial biochemical characterization of another kinase from Dictyostelium that is related to MHCK A. When the segment of this protein that is similar to the MHCK A catalytic domain was expressed in bacteria, the resultant protein displayed efficient autophosphorylation, phosphorylated Dictyostelium myosin II, and also phosphorylated a peptide substrate corresponding to a portion of the myosin II tail. We have therefore named this gene myosin heavy chain kinase B. These results provide the first confirmation that sequences in other proteins that are related to the MHCK A catalytic domain can also encode protein kinase activity. It is likely that the related segments of homology present in rat eukaryotic elongation factor-2 kinase and a putative nematode eukaryotic elongation factor-2 kinase also encode the catalytic domains of those enzymes.